1. Field of the Invention
The present invention relates to an electronic sphygmomanometer, and, more particularly, to an electronic sphygmomanometer arranged to be operated in accordance with the Korotkoff method and the oscillometric method.
2. Description of the Prior Art
In a blood pressure measuring method performed without watching blood, systolic blood pressure,diastolic blood pressure and mean blood pressure are measured based on a change of blood flow in an artery under a cuff which wraps on a blood pressure measuring portion, through a process of reducing the pressure of the cuff at a slow fixed speed after the pressurization of an expected cuff over the systolic blood pressure, or through a process of pressuring the cuff a fixed speed from a pressure less than the diastolic blood pressure to a pressure more than the systolic blood pressure. Such methods include the Riva-Rocci-Korotkoff method of measuring the blood pressure in accordance with a recognition of Korotkoff sounds (to be called the "K-sound method" hereinafter) and the oscillometric method of measuring the blood pressure in accordance with the oscillating pulse wave overlapping on the cuff pressure (to be called the "OSC method" hereinafter).
Conventional electronic sphygmomanometers have been arranged to act in accordance with only either method of the above-described methods: the K-sound method and the OSC method. Therefore, these electronic sphygmomanometers have included the advantages and the drawbacks of each method. For example, a problem arises in that an accurate blood pressure level cannot be obtained from persons who show too weak a K-sound to be detected, from persons having strong pulses and from patients who are being subjected to artificial dialysis and who therefore show strong artery shunt noise, causing the K-sound to be undesirably detected to the level below a diastolic blood pressure. On the other hand, the OSC method sphygmomanometer has a problem in that the blood pressure cannot be accurately measured from a person who shows too small oscillating pulse waves, when the cuff belt is wound too loosely, when a relatively large cuff is wound around a thin arm or when the blood pressure change is too excessive during the blood pressure measurement operation (the blood pressure change at breathing time).